U.S. Pat. No. 3,935,754 discloses a shaft failure indicator for indicating when a failure has occurred in a primary drive train 21. A secondary drive train 29 is provided which functions as the primary source of power for devices to be actuated when the primary drive line fails. A coupling unit 27 has a free zone of motion which prevents torque from being transmitted from a primary control unit 19 to the secondary drive train 29 unless a primary driveline failure has occurred. A failure indicator 104 is provided in the coupling unit 27 to detect when relative motion between the input shaft 25 and the secondary drive train 29 is equal to the free zone of motion which is indicative of a failure in the primary drive train 21. The movement of the drive shaft 25 with respect to the secondary drive train 29 through the free zone of motion causes movement of a member axially with respect to the axis of the drive shaft 25 to activate an indicator. In a second embodiment of the failure indicator, a pair of threaded control nuts engage a worm wheel which is driven by a worm gear 101 that is connected to the drive shaft 25. A difference in the velocity of the drive shaft 25 with respect to the secondary drive train 29 causes a displacement of the control nuts 115 in a direction parallel with the axis of the secondary drive train 29 to activate an indicator. The system of U.S. Pat. No. 3,935,754 does not have any mechanism for damping the rate of relative rotation of the drive shaft 25 with respect to the secondary drive train 29 in the free zone of motion.
U.S. Pat. No. 4,559,023 discloses a torque damper having a damping mechanism positioned concentrically with a driven shaft and in communication with a variable volume hydraulic chamber. A function of the system in U.S. Pat. No. 4,559,023 is to absorb transient torque loads.
The assignee of the present invention manufactured a system having a primary driveline and a backup driveline which was included in the Dash 8 aircraft sold by DeHavilland Corporation.